Studies were performed on rats and dogs to determine if prostaglandins mediate the vasodilator action of certain anti-hypertensive drugs. The prostaglandin synthetase inhibitor, indomethacin, had no effect on the anti-hypertensive action of hydralazine in spontaneously hypertensive rats but it reduced the anti-hypertensive action of hydralazine in our spontaneously hypertensive dogs. This suggests that prostaglandins may mediate some of the anti-hypertensive action of hydralazine in certain species of animals.